test(VDF): add bench tests for VDF in class group

[HAOYUatHZ]

This PR allows bench test VDF in class group by running

cargo bench --bench bench-vdf-class  --jobs 1

environment

• Ubuntu 20.04.1 LTS
• linux 5.4.0-48-generic
• cargo 1.44.1 (88ba85757 2020-06-11)
• rustup 1.22.1 (b01adbbc3 2020-07-08)
• rustc 1.44.1 (c7087fe00 2020-06-17)

how to test it

cargo bench --bench bench-vdf-class  --jobs 1

test result

eval with difficulty 10 time:   [11.250 ms 11.267 ms 11.286 ms]                                    
                        change: [+60.995% +62.097% +62.982%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 17 outliers among 100 measurements (17.00%)
  2 (2.00%) high mild
  15 (15.00%) high severe

verify with difficulty 10                                                                            
                        time:   [7.7320 ms 7.7783 ms 7.8527 ms]
Found 12 outliers among 100 measurements (12.00%)
  4 (4.00%) high mild
  8 (8.00%) high severe